1869, "relating to promotion or advancement," from promotion + -al (1). From 1902 as "relating to advertising."
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promotion n.
c. 1400, promocioun, "advancement in rank, honor, or position," from Old French promocion "election, promotion" (14c., Modern French promotion) and directly from Latin promotionem (nominative promotio) "advancement, a moving forward," noun of action from past-participle stem of promovere "move forward, advance" (see promote).
From early 15c. as "advancement of a cause; assistance, help, support, encouragement." The meaning "advertising, publicity" first recorded 1925.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).